The fundus cameras known from the prior art, by means of which artifacts of undesirable eye interfaces may be suppressed, may be divided into basically three optical device classes.
Classical wide-field fundus cameras which use annular pupil division as the most important feature may be included in the first group. While the ocular fundus is illuminated by an external illumination ring, the light that is scattered back from the fundus is detected through the circular center zone of the pupil of the eye, which is free of illumination light. To delimit the external illumination ring from the detection area in the center of the pupil of the eye, a transition zone approximately 1 mm wide in which neither illumination nor detection occurs is present between the two areas. This transition zone is necessary to achieve a complete separation of illumination beams and detection beams, not only in the corneal plane, but also in the entire anterior chamber of the eye, i.e., from the posterior side of the cornea to the anterior side of the lens of the eye. Although wide-field fundus cameras may be used to record reflection-free images of the ocular fundus due to the distinct separation of illumination and detection, the achievable fundus angle is limited by the annular pupil division, and reaches maximum values of approximately 45 degrees for pupil diameters of approximately 4-5 mm. Pupil diameters of 2 mm, which occur in non-dark-adapted eyes, are not possible.
A second group of fundus imaging devices includes ophthalmoscopes, of which primarily confocal scanning laser ophthalmoscopes (CSLO) and line-scanning laser ophthalmoscopes (LSLO) have become established. In a scanning laser ophthalmoscope, the ocular fundus is scanned by a focused laser beam, and the light scattered back from the retina is imaged through a detection aperture onto an image sensor. Due to the confocal detection, reflections and scattered light from various planes of the eye, for example from the cornea or the lens of the eye, are suppressed, and artifact-free images of the ocular fundus may be recorded.
Since line-scanning laser ophthalmoscopes (LSLOs) have a design similar to that of CSLOs, the properties of the latter correspondingly apply. In contrast to CSLOs, in LSLOs the ocular fundus is scanned by use of a laser line instead of a laser spot. However, the suppression of interference signals is generally not as good as in CSLOs. In return, LSLOs are less technically complex, less expensive, and easier to adjust to the eye to be examined.
However, if the CSLO is not optimally aligned with the eye to be examined, or if the eye to be measured differs from an average eye for which the CSLO has been optimized, during the suppression of the undesirable scattered light, portions of the light scattered back from the retina and intended for detection are also suppressed. For this reason, the CSLOs, which are generally quite expensive, are used primarily as “high end” devices which allow very high-quality fundus recordings, but which are much more difficult to adjust to the eye to be measured compared to, for example, the wide-field fundus cameras mentioned in the first group.
Another disadvantage of scanning laser ophthalmoscopes is that recording a color fundus image by the scanning process takes considerably longer than for wide-field systems. The motion artifacts which result, in particular in restless patients, additionally complicate and/or impair the recordings. Another limitation in the use of CSLOs is their high level of technical complexity and the resulting higher acquisition costs, for which reason they are seldom used in the “low end” sector.
The third group includes wide-field fundus cameras having transscleral illumination. In these fundus cameras, the retina is imaged on a camera sensor, utilizing the entire pupil of the eye. The illumination occurs through the sclera of the eye. A complete separation of the illumination beam path and the detection beam path may be ensured in this way. Despite these major advantages, fundus cameras having transscleral illumination have not become established on the market since their development by Pomeratzeff (1974).
To achieve an artifact-free image, in these fundus cameras an illumination optical fiber is placed directly on the sclera (in the area of the pars plana). Significant application-related limitations result from this direct contact between the illumination optical fiber and the eye. Thus, the illumination optical fiber must, for example, be cleaned and sterilized before each diagnosis.
Each of these three groups of fundus imaging devices has specific advantages as well as disadvantages, but none is able to meet all required specifications.